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Modem Communication

In bšrjan Poe 70s in the U.S. they fšrsta modems are PoE market.
FSR dialed fšrbindelser speed was lŒg, ² 300 bps and performance Šnnu lŠgre. A need existed to communicate between computers and telephone lines was the obvious media. Speeds Injury sŒsmŒningom and at the end of the 70s the DSK fšrsta modems are up FSR 1200 bps full-duplex SVER dial fšrbindelse. Unfortunately, there were standards established in the United States and Europe that were not compatible. An organization was formed early FSR to standardize various kommunikationsmoder.och procedures, CCITT.
CCITT has members Frøen all vŠrldens telefšrvaltningar and Sven de stšrre equipment manufacturers FSR data communications and recommendations are often designated V.nn describing in detail how the equipment to communicate with other equipment fšljer a specific recommendation.
One can SSGA that from that speed 2400 bps and uppŒt have a common standard is established, ie, V.22bis (2400 bps) and uppŒt. FSR to uppnŒ stšrre sŠkerhet SVER line developed several algorithms fšretag FSR ARQ, omsŠndning of erroneous data, and data compression. Market became Microcom, Inc., which developed the MNP protocols Sven tŠckte data compression.
CCITT is behind a recommendation benŠmnd V.42 FSR FSR ARQ and V.42bis compression. Today mŒste a leverantšr have bŒde MNP and V.42 implemented in their products FSR be konkurrenskraftiga.Fasta fšrbindelser, 2-trŒdar FSR sŠndning and 2-trŒdar FSR reception could lŠttare adapted to datašverfšring and speeds SVER line nŒdde already in the middle of the 70 - 's 9600 bps. 1974 erbjšd Codex, USA Pools FSR 9600 bps fixed fšrbindelse. Several competitors fšljde the nŠrmaste Oeren and 1978 faststŠlldes a recommendation within CCITT V.29 CCITT as specified by the standard solid fšrbindelse FSR FSR 9600 bps. Several standards fšljde FSR lŠgre speeds as wise FSR V.27 4800 bps.

Data rate and line speed.

The SR is important that we distinguish Poe concepts bps and baud. More of vŒra competitors do not separate PoE Boeda these concepts. Bps SR fšrkortning FSR bits per second and refers to the speed between the computer and modem. The speed SVER phone line listed in Baud and definition Sr. fšrŠndringar per unit time. PoE line communication is done with modulated tones and the bandwidth we have to fšrfogande SR 2400 Hz, a so-called voice channel. A telephone can not accept more SN 2400 Hz or 2400 fšrŠndringar per unit time (1 second). SŒledes mŒste we resort to some tricks FSR to šverfšra hšga bitrates Poe a line that has begrŠnsat space. This gšrs in modem sŠnder out coded signals, each signal DSR innehŒller information on a number of pieces.
FSR to šverfšra further hšgre speeds between computers kodkomprimering developed.

Modem Technology.

FSR lŠgre speeds, ² 2400 bps, parts modem line circuits up telephone line in a sŠndkanal and a receiver channel. These benŠmns ORG and ANS, the originate and answer. We šverfšr alltsŒ data in each direction PoE line wholly independently. At hšgre speeds 9600 and 14400, this technique can not anvŠndas. The capacity of the channels rŠcker simply not to. The technology developed is called Echo Cancelling and stŠller great demands Poe processing capability of the modem.
Put simply takes fšljande in communication between Tvoe Pools of type V.32 or V.32bis.

A modem dials and establishes fšrbindelse with modem B.
Modems were shaking hands.
Modem A sŠnder out a test signal that takes full fšrbindelsens capacity ansprŒk. Modem A mŠter what possible echoes upptrŠder PoE line and stores these vŠrden in its memory. Modem A finishes his test.
Modem B utfšr same procedure as modems A.

Boeda modems now have information about their memories karaktŠristik line and modem are sŠnder now while this information takes the whole band in ansprŒk. The information PoE line bestŒr now the information Frøen Boeda modems sŠndare and echoes Frøen line. Modems were talking in the mouth Poe each other!

How ex wise modem A know what SR information Frøen modem B?
Well, the information PoE line bestŒr of A's sŠndare, B's sŠndare and echoes Frøen line. If A deletes its own sŠnda information and the echoes that it stored in its memory mŒste the remaining information must be B's sŠnda data!
Error correction, compression and grŠnssnittshastighet.
A dial fšrbindelse SR often exposed FSR various stšrningar as innebŠr to šverfšrda data will be incorrect. By blocka data and fšrse the sŠnda blocks with checksum can any incorrect data is detected and omsŠndas. Tvoe different ARQ (Automatic Repeate reQuest) algorithms fšrekommer. First, MNP 4, and V.42. The SR is not compatible with each other but automatic choice can be made between them at speed modemfšrbindelsen.

FSR to uppnŒ stšrre efficiency and hšgre speeds at šverfšringen is accessed data compression. Tvoe different algorithms fšrekommer. First, MNP 5, V.42bis both. These algorithms can only compress data asynchronously. Compression of synchronous data krŠver kŠnnedom on the Fonts Used communication protocol. The BSR nŠmnas to V.42bis SR effective Sn MNP5. The SR is not compatible with each other but automatic choice can be made between them at speed modemfšrbindelsen.
In principle, based on these algorithms Poe to the most fšrekommande characters are encoded into a shorter and less frequent fšrekommande signs a lŠngre code. The ASCII alphabet, we have 1 start, 7 information bits, 1 stop bit and 1 parity. Total 10 bits. Start bit, stop bit and parity bit is removed by sŠndande modem and still we have 7 pieces of information. SŠndande modem analyzes information bits and ex way, we can encode the most fšrekommande characters

A 1111
T 0111
E 0011
S 0001
R 11001
U 01100
..
..
Z 111001011
to all lŠngre codes FSR less frequent characters.
In sŠndande modem built up a code table that fšrŠndras dynamic and šverfšres to receiving modem FSR decoding. The result of this coding gives a compression of a factor of 2 - 3. Ie we can sŠnda data to the modem with 2-4 gŒnger hšgre speed sn its hšgsta šverfšringshastighet. A factor 4 out sometimes, but fšrekommer in extreme cases only.

Buffered grŠnssnitt

FSR that the PC should receive varying speeds Frøen modem depending šverfšringshastighet krŠvs a buffered grŠnssnitt the modem fixed speed regardless šverfšringshastighet. GrŠnsnittet can usually stŠllas the FSR speeds of 2400, 4800, 9600, 19200, 38400, 57600 and 76800 bps. The tvoe last 57600 and SR 76800, in practice, we doe not anvŠndbara with V.32 Pools and uppnŒr a compression POE 2 - 3. At hšga speeds are Sven begrŠnsningar in PC hŒrd and software.

Standards FSR dialed fšrbindelser.

CCITT speed AnmŠrkning
V.21 300 asynchronous data
V.23 75-1200 Asynchronous data
These standards have nŒgon practical importance today, and lŒga speeds and dŒliga performance.

V.22 1200 Asynchronous and synchronous data
V.22bis 2400 Asynchronous and synchronous data
These standards SR POE VSG to ersŠttas of hšgre speeds, but they fšrekommer in a large number of Pools POE market. LŒga prices combined with compression GSR them still attractive.

V.32 9600 Asynchronous and synchronous data
V.32bis 14400 Asynchronous and synchronous data
A V.32 modem has always V.22 standard, built-in, one bed for the Poe kšpet and a V.32 modem automatically adapts to the called modem standards. Communication is established automatically hšgsta mšjliga speed.

Another standard, probably the last FSR dialed fšrbindelser, Sr. in preparation and will be fixed in the middle of 1984. The standard has during operation gŒng benŠmnts V.Fast and will NSR it determined that benŠmnas V.34. HSR SR hšgsta speed 28800 SVER line and with good compression, we may in some 100 000 Kbs. Great demands stŠlls however PoE line kvalitŽ.

Market demands Poe a modem dial-up FSR fšrbindelse.
HSR SR nŒgra concepts and features that Sr more or less a mŒste FSR the modem to be competitive:

T mŠrkning.

FSR to FOE connect equipment to Telecom's lines krŠvs the modem mšter certain requirements. These requirements SR uppstŠllda FSR not to connected equipment shall stšra telefonŠtets function or other subscribers who communicate SVER nŠtet. That a modem SR T-MŠrkt innebŠr not the modem has genomgŒtt prestandamŠtning or other PoE SSTT SR bŠttre Sn second Pools: T-mŠrkning innebŠr only the modem SR godkŠnt FSR connection to allmŠnna telefonnŠtet.

Automatic Dialing -

AT commands or Hayes compatible.
Hayes Inc. in the United States was fšrst to define a command set FSR Pools. Commands bšrjar always with the characters AT (ATtention) fšljt of characters FSR various functions, such as wise ATDT123456 relates D = Dial and T = tone, ie call the number 123456 with tonsignallering. Note there is no link between a mother's performance and functionality and to be Hayes AT compatible. It covers only the compatibility with the command set. Most modems have a utškat command set that is different modem in between.
FSR dialing with synchronous terminals, or about the same number always ringes this can be done by the modem monitors the status of DTR leader in the PC one and automatically dials a number fšrvalt NSR DTR goer HSG, called 108.1 dialing.
Another option Sr. to dial a number fšrlagrat Frøen front panel.

MNP4/V.42, MNP5/V.42bis & MNP10
These relate to the features error correction and compression. Especially compression may be more or less effectively implemented. This SR svŒrt FSR a customer to MSTA, DOE grŠnssnittshastigheten usually SR set at the hšgsta modem accepts and anvŠndaren think he šverfšr data in this rate; It seems that the data šverfšrs with grŠnssnittshastigheten but in practice SR effective šverfšringshastigheten often lŠgre. MNP10 developed FSR modemšverfšring via NMT or Mobitex as stŠller requirements Poe bibehŒllen synchronization at bŠrvŒgsfšrlust

Buffered grŠnssnitt.

Fast grŠnssnitthastighet regardless šverfšringshastighet sver line SR a mŒste. Ex wise SR speed between the PC and the modem is set to 38400 bps regardless of connection to the 14400 or 2400 modem.

Number list.

List of modem DSR anvŠndaren can store contact information for fast and easy dialing either by specifying the position in the list, or by mentioning names.

Automatic Œteruppringning.

FSR case the number Sr uptake is modem Œteruppringa up to nine gŒnger with 1 minute intervals, according to the Swedish tele Policy. The function SR often put out of action, DOE communications software can Œteruppringa almost immediately after the busy signal and repeat Œteruppringning in oŠndlighet.
Call back with lšsenord.
Answering modem avkrŠver calling modemanvŠndare lšsenord. Dial-up modem controls specified lšsenord against a fšrlagrad list. A lšsenord are in the list, Sven has a phone DSR the caller can Noes. When properly specified lšsenord, disconnect dial-up modem down fšrbindelsen and motringer to the number in the list ..
FSR to a hacker or other otillbšrlig person must be able to establish fšrbindelse krŠvs partly to rŠtt lšsenord out and that that person is Poe rŠtt phone. Indication of incorrect lšsenord medfšr teardown.
In more extensive installations with multiple defendants modem is often installed as Network Management under software control gives statistics Poe number calling anvŠndare, incorrectly stated lšsenord etc

FjŠrrkonfigurering.

Dial-up modems is via sŠrskilt lšsenord able to monitor and configure fjŠrrŠndens parameters. This saves time and the FSR all costs associated with problemsškning and service calls.

FAX communication.

Moenga Pools can sŠnda today and receive FAX. The modem will stšdja Group 3 fax machines, ie, 9600 bps.

SNA communications.

In Sweden, sr 3270 SNA communications widespread and minicomputer systems AS 400 with 5250 communication a sŠljsuccŽ. Communication SVER dial fšrbindelse in these miljšer SR complex and krŠver either plug-in card in the PC FSR synchronous communication or connection with protocol converter. A third alternative is available with special software in your PC and an LTI modem.

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